1. Field of the Invention
The present invention relates in general to a hydraulically operated braking system, and more particularly to a control device for controlling a seating valve included in the braking system.
2. Discussion of the Related Art
JP-A-9-175375 (laid-open publication of Japanese Patent Application No. 7-336642 filed by the assignee of the present application) discloses a hydraulically operated braking system which includes (a) a brake for braking a wheel of a motor vehicle, (b) a seating valve, and (c) a seating valve control device for controlling the seating valve. The brake is provided with a wheel brake cylinder which is activated by a pressurized working fluid supplied thereto, to brake the vehicle wheel. The seating valve is provided in a fluid passage connected to the wheel brake cylinder, and includes a valve seat, a valve member which is movable toward the valve seat to be seated thereon and away from the valve seat, a biasing means for biasing the valve member in a biasing direction toward the valve seat, and a valve driving device which generates a drive force which acts on the valve member in a direction opposite to the biasing direction. The seating valve control device controls the valve driving device of the seating valve, so as to selectively open and close the seating valve for regulating the fluid pressure in the wheel brake cylinder. When the valve member is moved by the drive force generated by the valve driving device, away from the valve seat against the biasing force of the biasing means, the seating valve is placed in an open state thereof. When the drive force is zeroed, the valve member is seated on the valve seat by the biasing force of the biasing means, so that the seating valve is placed in a closed state thereof.
In the hydraulically operated braking system disclosed in JP-A-9-175375 (which is not qualified as the prior art against the present invention), the fluid pressure in the wheel brake cylinder is regulated by two seating valves, namely, a pressure-increasing seating valve and a pressure-reducing seating valve. The pressure-increasing seating valve is disposed between the wheel brake cylinder and a master cylinder, and is adapted to be opened to increase the fluid pressure in the wheel brake cylinder. The pressure-reducing seating valve is disposed between the wheel brake cylinder and a reservoir, and is adapted to be opened to reduce the fluid pressure in the wheel brake cylinder.
However, the braking system described above suffers from a problem that the seating valve generates a relatively large impact upon seating of the valve member onto the valve seat. Since the valve member is biased toward the valve seat, the valve member is seated onto the valve seat at a high velocity, with a large impact therebetween, when the drive force generated by the valve drive device is zeroed. The large impact results in an accordingly large noise of abutting contact between the valve member and the valve seat, causing deterioration of driving comfort of the vehicle. Further, the high seating velocity causes early wear or fatigue of the contacting portions of the valve member and valve seat, leading to reduced durability of the seating valve.